Ink or coating compositions applied on transparent or semi-transparent substrates conventionally are exposed to radiation curing only from one surface of the substrate. Generally, the surface with the composition applied thereon directly is exposed to radiation. One-sided radiation exposure affects the extent of polymerization.
Non-uniform polymerization may result from decreased light intensity in the z-direction of a substrate. In accordance with the Beer-Lambert Law, logarithmic dependence exists between the transmission, T, of light through a substance and the product of the absorption coefficient of the substance, α, and the distance the light travels through the material (i.e., the path length), l. For liquids, transmission of light is defined as:
      T    =                  I                  I          0                    =                        10                      -            αℓ                          =                  10                                    -              ɛℓ                        ⁢                                                  ⁢            c                                ,wherein ε is a molar absorptivity (i.e., extinction coefficient) of the absorber, c is the concentration of absorbing species in the material, I0 and I are the intensity or power of the incident light and the transmitted light,
Free radical polymerization induces shrinkage when C═C bonds react with each other to form a polymer. Ink or coating compositions proximal to the radiation source typically shrink more than compositions located distal to the radiation source due to absorption diffusion and diffraction characteristics of radiation. In addition, monomers of the composition in contact with the top surface of the substrate are more likely to react with radicals from the substrate surface layers already chained to the cured surface layers than to react with radicals from the bottom surface of the substrate with a smaller concentration of radicals. Thus, non-uniform polymerization causes the coating or ink compositions to shrink from the edges toward the center of the substrate as well as from the bottom, non-applied composition surface towards the applied composition surface. Therefore, it is common to observe a thick layer of ink or coating warp after curing as shown in FIG. 1. Accordingly, these layers are prone to popping off and/or becoming separable from the substrate.
In the past decade or so, there has been an increased demand by manufacturers to improve adhesion characteristics between applied coating or ink compositions and substrates with high glass transition temperatures, Tg, high crystalline densities and/or high tensile strengths. This is primarily attributed to the tendency of cured composition layers to pop off the substrate. Primer or chemically treated layer or layers applied onto a substrates in order to lower the substrate's Tg and/or crystalline density. While adhesion properties may be improved, a significant increase in material cost is expected. Moreover, additional processing steps and equipment would be required.
Adhesion promoters also have been used to improve adhesion characteristics. However, adhesion promoters pose similar burdens as discussed above for primer layers or chemically treated layers. In addition, adhesion promoters are not user-friendly and may possibly lead to skin and eye irritations. Adhesion promoters also are prone to migration and therefore result in toxicity issues. Moreover, adhesion promoters include lower functional monomers and are less likely to be locked into the polymer backbone, especially near the bottom of the ink layer where radiation intensity is significantly weaker than at the surface of the ink layer. This affects curing speed.
A need therefore exists in the art for improving adhesion performance of coating and ink compositions applied on substrates with high Tg or high crystalline density (high tensile strength).
A need also exists for improving the curing speed of ink and coating compositions applied on substrates.
A need further exists for a product with improved adhesion and/or cure properties.